1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More specifically, the invention is in the field of fabrication of transistors.
2. Related Art
By utilizing BiFET technology, bipolar transistors, such as heterojunction bipolar transistors (“HBTs”), and field effect transistors (“FETs”) can be integrated on the same semiconductor die to provide devices, such as RF power amplifiers, having increased design flexibility. As a result, a BiFET power amplifier including an HBT and a FET can be advantageously designed to operate at a lower reference voltage than a bipolar transistor power amplifier. Of particular interest to device manufacturers are high power BiFET amplifiers, which can be formed by integrating a FET into a gallium arsenide (“GaAs”) HBT process. However, previous attempts to integrate a FET into a GaAs HBT process have resulted in degraded HBT performance and/or reduced FET manufacturability.
For example, in one conventional approach, a FET can be formed using a GaAs emitter cap layer as a FET channel, which is situated between an aluminum gallium arsenide (“AlGaAs”) emitter layer and a heavily doped N type GaAs layer. A recess can be formed in the heavily doped N type GaAs layer by utilizing a timed etch process and a gate layer can be formed in the recess. However, as a result of the timed etch process, FET threshold voltage uniformity is difficult to achieve in the above approach, which decreases FET manufacturability.
In an effort to avoid using a timed etch process, an aluminum arsenide (“AlAs”) etch stop layer has been utilized over the channel layer in a FET formation process. However, when an AlAs etch stop layer is utilized to form a BiFET including a FET and a GaAs HBT, the AlAs etch stop layer degrades HBT performance by undesirably blocking electron flow in the HBT. Furthermore, since oxidation of the AlAs etch stop layer can cause portions of the device situated over the AlAs etch stop layer to break off, the AlAs etch stop layer reduces long term device reliability.
Thus, there is a need in the art for a BiFET that achieves increased FET manufacturability without causing degradation in HBT performance.